Mechanics of Collagen in the Human Bone: Role of Collagen-Hydroxyapatite Interactions
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Mechanics of collagen in the human bone: role of collagenhydroxyapatite interactions Shashindra M. Pradhan, Kalpana S. Katti and Dinesh R. Katti Department of Civil Engineering, North Dakota State University, Fargo ND 58105USA ABSTRACT Here, we report results of our simulations studies on modeling the collagenhydroxyapatite (HAP) interface in bone and influence of these interactions on mechanical behavior of collagen through molecular dynamics and steered molecular dynamics (SMD). Models of hexagonal HAP (10 1 0) and (0001) surface, and collagen with and without telopeptides were built to investigate the mechanical response of collagen in the proximity of mineral. The collagen molecule was pulled normal and parallel to the (0001) surface of hydroxyapatite. Water molecules were found have an important impact on deformation behavior of collagen in the proximity of HAP due to their large interaction energy with both collagen and HAP. Collagen appears stiffer at small displacement when pulled normal to HAP surface. At large displacement, collagen pulled parallel to HAP surface is stiffer. This difference in mechanical response of collagen pulled in parallel and perpendicular direction results from a difference in deformation mechanism of collagen. Further, the collagen molecule pulled in the proximity of HAP, parallel to surface, showed marked improvement in stiffness compared to absence of HAP. Furthermore, the deformation behavior of collagen not only depends on the presence or absence of HAP and direction of pulling, but also on the type of mineral surface in the proximity. The collagen pulled parallel to (10 1 0) and (0001) surfaces showed characteristically different type of load-displacement response. In addition, here we also report simulations on 300 nm length of collagen molecule indicating the role of length of model on the observed response in terms of both the magnitude of modulus obtained as well as the mechanisms of response of collagen to loading. INTRODUCTION Collagen, the most abundant protein in the human body, is a structural protein that constitutes major portion of total bone protein. In human bone, the weight percentages of organic mineral phase, and water are about 25%, 65%, and 10% respectively 1-3. The collagen molecule is a triple helical structure consisting of three poplypeptide chains (called α chains) wound into a right hand helix, and measures about 300 nm in length and 1.3 nm in diameter4. The polypeptide chains are made up of repeating triplets of (Glycine-X-Y), where X and Y are other amino acids. These three chains are held together by hydrogen bonds. In bone, the collagen molecules assemble in conjunction with mineral, water and other organics to form fibrils5.
In this study, we have investigated directional dependence of collagen when pulled parallel and perpendicular to the mineral surface. The deformation behaviors of collagen in the proximity of different mineral surfaces are also studied. We have used (10 1 0) and (0001) surfaces of hydroxyapatite for our study. T
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